The invention relates to a high power pulsed electron beam system, and more particularly to a system for producing a pulsed, high current, electron beam from a hot cathode for annealing semiconductor devices at some point during their fabrication.
During the past few years, the use of electron radiation as a heat source for annealing ion-implantation damage in Si and GaAs has shown dramatic progress. With electron beam annealing, a higher degree of crystalline perfection can be achieved than with furnace annealing. But even if that were not the case, there are still numerous advantages over thermal heating. Surface annealing to any desired depth may be quickly achieved, and because only the surface of the semiconductor device requires annealing while still part of a wafer, there is less possibility of warping.
Crystalline defects within an implanted layer are eliminated if the depth of the melted layer exceeds that of the damaged surface layer and rapid growth occurs on an underlying single crystal substitute. It is the melting and subsequent rapid growth that is responsible for achieving improved crystalline perfection by annealing with a pulsed beam. The beam heats the crystal only to the desired depth, while the rest of the environment, including the crystal holder, remains cool. A pulsed beam annealer must therefore provide controlled beam energy density for uniform melting the surface layer to a desired depth.
The appropriate energy density required for pulsed annealing can be calculated, or determined empirically. In general, the energy density required for such annealing is related to the melting point of the semiconductor material and the depth of the layer to be melted. Since the depth is an important parameter, it is necessary to control the energy density. This can be done to a first order to magnitude by controlling the electron beam density, but for a higher order of control, it is necessary to also control the velocity of the electrons. It is therefore important to produce a controlled, high density beam that may be pulsed for fast, shallow heating with an electron beam having a linear wavefront that is uniform over the crystal surface, followed by fast cooling. The latter requires localized heating of only the crystal surface, and not the crystal holder and its environment.